It is generally known that a veneer sheet produced, e.g., by peeling tends to be curled or become wavy and such curling and waviness make it troublesome to handle the sheets. It is thus desirable to make the sheet substantially flat by a process known as tenderizing by which a number of checks, or small cracks, are formed in the veneer sheet by incising the same with cutting means such as blades so as to wedge the veneer apart in the direction perpendicular to the veneer grain or fiber orientation. It is also known that the tenderizing of wood veneer sheet has many other helpful advantages such as expansion of the sheet in the direction perpendicular to the grain, hence an increase in the veneer yield, faster drying due to easier flow of hot air through the tenderized sheet, thereby saving the cost in veneer drying, etc.
A veneer sheet tenderizing device of a well known type comprises a pair of rolls spaced so as to form a nip therebetween through which a veneer sheet is fed with its grain directed substantially perpendicular to the direction in which the sheet is fed. At least one of the nip rolls has on its periphery a number of projections the distal ends of which are defined by an edge having a given length and extending substantially in parallel to the grain of veneer sheet being fed into the nip and the edge is adapted to wedge the wood apart across the grain thereby to form checks in the sheet in the grain direction.
The checks can be formed successfully if the grain orientation coincides with, or angularly offset only slightly, e.g. at about 8 degrees, from, the extending direction of the projection edge. However, if an angle of the grain orientation with respect to the extending direction of the edge is greater, e.g. at about 10 degrees, the veneer sheet at the point of engagement with the edge is only crushed without being wedged apart and, as a result, the desired tenderizing effect cannot be achieved. Such poor tenderizing effect is noticeable particularly in such veneer sheets that are peeled from a block of low grade having its wood grain running in various directions, e.g. a peeler block available from part of a log which is relatively close to the root portion of a tree and therefore has a tangle of wood fibers extending in varying directions. If a poorly tenderized veneer sheet is dried, it will be warped to such an extent that makes the sheet unusable.
A tenderizing device designed in an attempt to solve the above problem has been proposed by the Publication of Examined Japanese Utility Model Application No. 50-34312 (1975), wherein its tenderizing roll has formed on the periphery thereof a number of projections disposed at regular intervals along a plurality of zigzag lines each extending generally in the axial direction of the roll with alternating angles and spaced from each other at regular intervals in the circumferential direction. The distal end of each projection is defined by a straight edge having a given length and oriented in alignment with a straight line section of the zigzag line, so that the projections are formed with two different inclinations for their straight edges with respect to the roll axis.
With use of such a tenderizing roll, checks can be formed satisfactorily where the grain direction happens to substantially coincide with the inclination of the projection edges, while such checks cannot be produced where an angle between the edge inclination and the grain direction is large.